Hi there and welcome!
The best way I know how to tell by is by looking at the seal of the bubble to the card.
If it is a Kenner 12 back, then you should be able to see a distinct "waffle pattern" between the bubble and card.
Joe_O has a write up here on TIG that mentions it, but the image is gone for some reason.
here is the LINK TO THAT THREAD
However, open a browser and go to images.google.com and search on this exact term:star wars waffle pattern
and the image that was supposed to be in the post should be the first image that Google shows you.
If you can't see the waffle pattern (I assume you are looking at one to buy), ask the seller for more images close up of that area.
To be honest, the waffle pattern is the only way I know on the Kenner cards, but perhaps there are more.
If you don't see a waffle pattern or cannot tell, then it is tough to say how to confirm it.
Look all around the seal of the bubble for either horizontal or vertical lines near where the tops and sides of the bubble seal meet. If you do, it may indicate there is tape there.
Here is one image of where you can see the tape lines (in this case they are horizontal, but they can go vertical).
You will also notice no waffle pattern there since it is just double sided tape.
The lines are caused by the seam where the side and top pieces meet.
Seeing lines in that area is not absolute (like anything).
Anyway... I hope that helps you a bit.